The LLVM Lexicon¶
This document is a work in progress!
- Aggressive Dead Code Elimination
Abstract Syntax Tree.
Due to Clang’s influence (mostly the fact that parsing and semantic analysis are so intertwined for C and especially C++), the typical working definition of AST in the LLVM community is roughly “the compiler’s first complete symbolic (as opposed to textual) representation of an input program”. As such, an “AST” might be a more general graph instead of a “tree” (consider the symbolic representation for the type of a typical “linked list node”). This working definition is closer to what some authors call an “annotated abstract syntax tree”.
Consult your favorite compiler book or search engine for more details.
- BB Vectorization
- Basic-Block Vectorization
- Bottom Up Rewriting System — A method of instruction selection for code generation. An example is the BURG tool.
- Call Frame Information. Used in DWARF debug info and in C++ unwind info to show how the function prolog lays out the stack frame.
- Common Information Entry. A kind of CFI used to reduce the size of FDEs. The compiler creates a CIE which contains the information common across all the FDEs. Each FDE then points to its CIE.
- Common Subexpression Elimination. An optimization that removes common
subexpression compuation. For example
(a+b)*(a+b)has two subexpressions that are the same:
(a+b). This optimization would perform the addition only once and then perform the multiply (but only if it’s computationally correct/safe).
- Directed Acyclic Graph
- Derived Pointer
- A pointer to the interior of an object, such that a garbage collector is unable to use the pointer for reachability analysis. While a derived pointer is live, the corresponding object pointer must be kept in a root, otherwise the collector might free the referenced object. With copying collectors, derived pointers pose an additional hazard that they may be invalidated at any safe point. This term is used in opposition to object pointer.
- Data Structure Analysis
- Dead Store Elimination
- First Class Aggregate
- Frame Description Entry. A kind of CFI used to describe the stack frame of one function.
- Garbage Collection. The practice of using reachability analysis instead of explicit memory management to reclaim unused memory.
- In garbage collection, the region of memory which is managed using reachability analysis.
- Inter-Procedural Analysis. Refers to any variety of code analysis that occurs between procedures, functions or compilation units (modules).
- Inter-Procedural Optimization. Refers to any variety of code optimization that occurs between procedures, functions or compilation units (modules).
- Instruction Selection
- Loop-Closed Static Single Assignment Form
- “Looks Good To Me”. In a review thread, this indicates that the reviewer thinks that the patch is okay to commit.
- Loop Invariant Code Motion
- Language Specific Data Area. C++ “zero cost” unwinding is built on top a generic unwinding mechanism. As the unwinder walks each frame, it calls a “personality” function to do language specific analysis. Each function’s FDE points to an optional LSDA which is passed to the personality function. For C++, the LSDA contain info about the type and location of catch statements in that function.
- Load Value Numbering
- Link-Time Optimization
- Machine Code
- “No functional change”. Used in a commit message to indicate that a patch is a pure refactoring/cleanup. Usually used in the first line, so it is visible without opening the actual commit email.
- Object Pointer
- A pointer to an object such that the garbage collector is able to trace references contained within the object. This term is used in opposition to derived pointer.
- Problem report. A bug filed on the LLVM Bug Tracking System.
- Partial Redundancy Elimination
Replace All Uses With. The functions
Constant::replaceUsesOfWithOnConstant()implement the replacement of one Value with another by iterating over its def/use chain and fixing up all of the pointers to point to the new value. See also def/use chains.
- Rearranging associative expressions to promote better redundancy elimination
and other optimization. For example, changing
(B+A-A), permitting it to be optimized into
- In garbage collection, a pointer variable lying outside of the heap from which the collector begins its reachability analysis. In the context of code generation, “root” almost always refers to a “stack root” — a local or temporary variable within an executing function.
- Reverse postorder
- Safe Point
- In garbage collection, it is necessary to identify stack roots so that reachability analysis may proceed. It may be infeasible to provide this information for every instruction, so instead the information may is calculated only at designated safe points. With a copying collector, derived pointers must not be retained across safe points and object pointers must be reloaded from stack roots.
- Selection DAG Instruction Selection.
- Strongly Connected Component
- Sparse Conditional Constant Propagation
- Superword-Level Parallelism, same as Basic-Block Vectorization.
- Scalar Replacement of Aggregates
- Static Single Assignment
- Stack Map
- In garbage collection, metadata emitted by the code generator which identifies roots within the stack frame of an executing function.
- Type-Based Alias Analysis